How Contreras altered his photograh. Click on the image to see the image before and after digital manipulation. Photograph: Narciso Contreras/AP

The decision of the newswire Associated Press to 'sever its relationship' with the Pulitzer prize-winning freelance photographer Narciso Contreras for editing a picture sounds harsh, possibly even an over-reaction. Narciso is a good photographer who has produced some great images from the most dangerous of places, including Syria. All he did was clone a video camera out of the bottom corner of a seemingly pretty unimportant photograph. And he admitted his misdemeanour to the agency editors.

Sacking someone, albeit a freelance, seems very draconian. It was a first offence after all – AP has carefully checked all Contreras's 494 other photographs on their archive. A warning would have been more suitable, surely?

This image by Contreras of a rebel fighter in Aleppo was one in a series of 20 by AP photographers that won the 2013 Pulitzer Prize. Photograph: Narciso Contreras/AP

Except that the major wire agencies and their clients rely on their images being totally authentic; that's why news organisations like the Guardian spend many thousand of pounds each year on their contracts. In a news environment it's all about a chain of trust: from the photographers through to the agencies, newspapers and websites, and then to the readers. If that chain is broken, any picture could be suspect, and that can't be allowed to happen.

The Guardian has the same guidelines for news photography: no cloning, no retouching. The sad irony for Contreras is that if he had just cropped his image, everyone would be happy and he would still have his contract.

A rebel fighter fires heavy machine gun from the back of a pickup truck in the Jedida district of Aleppo, Syria. Photograph: Narciso Contreras/AP

AP has its own rules for a good reason, and it is made very clear to all photographers that any manipulation is not tolerated. Photographers have been sacked in the past and I'm afraid the right decision was made in this case – for photojournalism's greater good.